Elevator systems have proven useful for carrying passengers among various levels of buildings. Different building types present different challenges for providing adequate elevator service. Larger buildings that are more populated typically require increased elevator system capacity, especially at peak travel times. Different approaches have been suggested for increasing elevator system capacity.
One approach includes increasing the number of shafts or hoistways and elevator cars. This approach is limited because of the increased amount of building space required for each additional elevator. Another proposal has been to include more than one elevator car in each hoistway. Such arrangements have the advantage of increasing the number of cars without necessarily increasing the number of hoistways in a building. One of the challenges associated with systems having multiple cars in a single hoistway is maintaining adequate spacing between the cars and ensuring that they do not interfere with each other.
Another suggested approach has been to utilize a double deck elevator car in which two cabs are supported on a single frame in a manner that they both move in the elevator hoistway together. In some versions, the cabs can move relative to each other within the frame to adjust spacing between the cabs. Double deck elevators typically have heavier cars that require larger or more ropes, larger counterweights and larger motors. Each of these undesirably increases the cost of the system.